U.S. Pat. No. 4,775,853 (abstract) discloses a device and installation provided for the instantaneous and simultaneous detection, inside and outside, of radiations emitted in the infrared, visible and ultraviolet spectra by simultaneous physical phenomena having a character of risk, such as intrusion, fire, explosion, leaks of dangerous fluids and electric leaks, disturbances and absence of movement of a regular periodic phenomenon, said radiations being emitted directly by the phenomena to be monitored at the time when the risk appears or being caused artificially by directing over an appropriate field of view, in which take place said phenomena, a source of radiation comprised in the infrared, visible and ultraviolet, and adapted to the nature of the phenomena involved, said field of view covered by the detection device (video camera) having appropriate horizontal and vertical dimensions comprising at least one spectral correction filter with known pass band chosen as a function of the nature of the radiation, a linear or circular polarization filter, a microprism array, and an image booster.
Flame detectors of the above-cited type have been known for a long time. They are provided for the detection of open fire or glowing embers with the characteristic modulated emissions thereof as well as for outputting an alarm within a few seconds. For special applications, an alarm activation within a fraction of a second is also possible. With regard to the signal processing, flame detectors of said type are fine-tuned to the characteristic flicker frequencies of open fire, that is to say of flames and glowing embers, in the infrared range and, where appropriate, in the visible and ultraviolet range.
The known flame detectors have at least one first radiation sensor which is sensitive to infrared radiation in the 4.0 to 4.8 ?m wavelength range. Infrared radiation of said type is typically produced during the combustion of carbon and hydrocarbons. They can also have a further radiation sensor which is sensitive to characteristic emissions of metal fires in the UV range.
For outdoor applications, known flame detectors usually also have a second radiation sensor which is sensitive e.g. to infrared radiation in the 5.1 to 6.0 ?m wavelength range. Typically, this infrared radiation is stray radiation, such as e.g. infrared radiation from hot bodies, sunlight or radiation not originating from combustion processes of carbon and hydrocarbons. On the basis of the two sensor signals, an evaluation is then possible to determine whether open fire is involved or not in this case.
The above-cited flame detectors are typically aligned to a fire-safety critical region requiring to be monitored. This region may contain e.g. an internal combustion engine, a fuel depot or a raw materials warehouse.
In the vicinity of bodies of water and in particular on ships, it cannot be ruled out that reflected sunlight will also impinge on such a flame detector. The areas in the vicinity of such bodies of water are in particular ships, offshore drilling platforms, petrochemical plants or inshore or shoreline refineries, such as e.g. container ports. Ships include e.g. container ships, ferries, frigates or cruise ships. Particularly at these locations, fire constitutes one of the greatest hazards of all.
A problem in this case is the sporadic triggering of false alarms when the sun is low in the sky. The triggering of an alarm typically initiates an automatic request for an unnecessary, expensive and disruptive deployment of large numbers of fire fighters.
This problem is caused by the modulation of the reflected sunlight by the swell of the body of water in the flicker frequency range of the flame detector, i.e. in the frequency range from 8 to 20 Hz. It is in fact known to place e.g. a PE film (PE standing for polyethylene) or a wire mesh as a prefilter in front of the flame detectors or radiation sensors in order primarily to reduce the intensity of the incident reflected sunlight. In spite of this, false alarms in the event of unfavorable swell cannot be avoided in this case.
Furthermore, flame detectors having a broadband photocell with a daylight filter, such as e.g. in the 0.7 to 11 ??m wavelength range, are known from the prior art. The signal resulting from said photocell serves principally for adjusting the sensitivity of the aforementioned radiation sensors as well as the operating thresholds for the alarm activation in order to avoid an override caused in particular by directly incident sunlight. This additional sensor is also unable to prevent a false alarm.